One of such light source apparatuses is a bullet shaped LED lamp as shown in FIG. 24. The LED lamp may employ a single LED or an array of LEDs mounted on a lead frame 31.
The bullet shaped LED lamp is commonly fabricated by die bonding an LED chip 2 into a recess 31a provided in a metal lead frame 31 using a die bonding paste 7 such as silver paste or epoxy resin. Then, the electrodes (not shown) provided on the LED chip 2 are wire bonded to the lead frames 31 and 32 by bonding wires 9 of a thread metal such as gold. Finally, the LED chip 2, the lead frames 31 and 32, and the bonding wires 9 are encapsulated in a light transmissible sealing resin 33.
The sealing resin 33 is generally an epoxy resin material. The sealing resin 33 has three major functions. The first is a protective function. The sealing resin 33 protects the internal components from mechanical damage as well as the LED chip 2 from water. The second function is to improve the light pickup efficiency. As the LED chip 2 has as a high index of refraction as about 2.8, its interface with the air may introduce total internal reflection resulting from a difference in the refraction index. This will decline the light pickup efficiency of the LED chip 2. The LED chip 2 can thus be coated at the surface with the epoxy resin which has a refraction index of about 1.8 for minimizing the total internal reflection and increasing the light pickup efficiency. The third is a light controlling function. The sealing resin 33 provides an effect of lens at the surface by which the light emitted from the LED chip 2 can be converged or scattered. This effect can thus control the light.
Also, the lead frames 31 and 32 have three functions. The three functions are to support the LED chip 2 by acting as a bed during the die bonding of the LED chip 2, direct the light of the LED chip 2 towards the front with their sides at the recess 31a, where the LED chip 2 is die bonded, serving as mirrors, and dissipate heat from the LED chip 2 to the outside by thermal conductivity of a radiator plate 30.
The LED chip 2 remains high in the light emitting efficiency when the temperature is low in a normal range of use as a lighting device. Its light emitting efficiency will decline when the temperature is high. This is because the grid oscillation is promoted by a rise in the temperature thus increasing the non-radiant coupling between electrons and holes. The LED chip 2 may be a major heat generating component in the common LED light source apparatus. It is hence an important task for improving the light emitting efficiency of the LED chip 2 to readily dissipate heat generated by the LED chip 2 and decline the temperature of the LED chip 2.
Also, when its temperature rise is declined by increasing its effect of heat radiation, the LED chip 2 can be driven by a higher rate of input current in the forward direction. The higher the current, the more the emission of light from the LED chip 2 can be increased. Also, as the effect of heat radiation is improved, the operating life of the LED chip 2 can increase.
The operating life of the LED chip 2 may be increased by two aspects. The first aspect is now explained. Assuming that the operating life of a red light emitting diode ends up when the flux of its emitting light is declined to about 70% of the initial level, the LED chip 2 may last substantially sixty thousands hours. However, when the LED chip 2 is energized with a forward direction current greater than the rated level and thus driven under overload conditions, its performance will acceleratedly be declined by its own heat. The LED chip 2 can thus be prevented from increasing the temperature by improving the effect of heat radiation. Accordingly, the operating life of the LED chip 2 will hardly be declined but increased.
The second aspect is explained. In general, blue light emitting diodes or white light emitting diodes where blue light emitted by blue LEDs is converted into white light are higher in the energy of emitted light than red light emitting diodes. This causes the sealing resin 33 encapsulating the LED chip 2 to be gradually deteriorated and tinted to brown by the energy of emitted light from the LED chip 2. More specifically, once having been tinted, the sealing resin 33 may absorb more blue light and its tinting will be accelerated. Finally, the sealing resin 33 encapsulating the LED chip 22 is turn to brown. Accordingly regardless of the flux of emitted light from the LED chip 2 remaining at its almost initial level, the intensity of light passing through the sealing resin 33 will significantly be declined. Since the sealing resin 33 is susceptible to tinting to brown, the actual operating life of the LED chip 2 using a blue LED or a white LED, which is determined by the flux of light measured at the outside, is as low as about 6000 hours which is far shorter than that of any red light emitting diode. The tinting to brown of the sealing resin 33 is caused by a photochemical reaction. It is generally known that the speed of the photochemical reaction will be accelerated when the temperature of the sealing resin 33 rises up. Accordingly, the operating life of the LED chip 2 can successfully be improved by increasing the effect of heat radiation from the LED chip 22 thus to decline the temperature of the LED chip 22 and the sealing resin 33 and minimize the tinting to brown of the sealing resin 33 derived from the emission of light from the LED chip 2.
It is hence very important for the light source apparatus with LED, in view of improving the efficiency of light emission, increasing the intensity of light output, and extending the operating life, to improve the effect of heat radiation from the LED chip 2 to the outside. The bullet shaped LED lamp has two passages for dissipating heat from the LED chip 2 via the lead frame 31 to the radiator plate 30 and via the sealing resin 33 to the air. However, the passage of dissipating through the sealing resin 33 may fail to provide a desired level of the heat radiation effect because the thermal conductivity of the epoxy resin is rather low. Accordingly, the passage of dissipating heat via the lead frame 31 is preferably chosen. Yet, this passage is as long as 7 to 10 mm with the lead frame 31 arranged too narrow and its assisted heat radiation effect may stay unsatisfactory although is greater than the heat radiation effect with the passage through the sealing resin 33. As the result, the conventional light source apparatus finds it difficult to improve the effect of heat radiation.
For improvement of the heat radiation effect, a modification of the light source apparatus is disclosed in Japanese Patent Laid-open Publication (Heisei)1-311501. The modification is illustrated in FIG. 25. The modified light source apparatus has LED chips 92 mounted on a metal base printed circuit board 91 which is shaped by press forming. More specifically, the printed circuit board 91 is fabricated by providing an insulating layer 913 of e.g. an epoxy resin and a layer of wiring copper foil on a thin metal substrate 912 of e.g. aluminum, etching the wiring copper foil layer to form a wiring pattern 915, and press forming the assembly to shape a desired number of recesses 911. Then, the LED chip 92 is die bonded to the wiring pattern 915 on the insulating layer 913 at each recess 911. The LED chip 92 is electrically connected at its surface electrode to another end of the wiring pattern 915 which is isolated from the die bonded end of the wiring pattern 915. Finally, the recess 911 is filled with a sealing resin 96 to complete the modified light source apparatus.
The modified light source apparatus has the LED chips 92 die bonded by a die bonding paste to the wiring pattern 915. This allows heat released from the LED chips 92 to propagate to the die bonding paste, the wiring pattern 915, the insulating layer 913, and the metal substrate 912. The heat is then dissipated throughout the metal substrate 912. Accordingly, the heat dissipating passage is shorter than that of the conventional bullet shaped LED lamp and the efficiency of heat radiation can be improved.
However, the heat dissipating passage of the modification still includes the die bonding paste, the wiring pattern 915, and the insulating layer 913 which may interrupt the effect of heat radiation. The die bonding paste is commonly a silver particles dispersed resin material where the silver particles increase the thermal conductivity as compared with a simple resin composition and the overall thickness is as small as several tens micrometers and can scarcely interrupt the effect of heat radiation. The wiring pattern 915 consists mainly of the copper plated layer and its thermal conductivity is high enough to hardly decline the effect of heat radiation. The insulating layer 913 contains a ceramic filler dispersed into the resin base and its thermal conductivity is lower than that of the metal layers. Also, the insulating layer 913 is as great as 300 micrometers in the thickness and can thus interrupt the effect of heat radiation.
The modified light source apparatus having the arrangement shown in FIG. 25 is higher in the effect of heat radiation from the LED chip 92 than the conventional bullet shaped LED lamp but fails to improve the effect of heat radiation because its insulating layer 913 interrupts the heat dissipating passage.
When the LED chip has both p electrode and n electrode on one side thereof, it can directly be joined by die bonding to the insulating layer. There is still the insulating layer which declines the thermal conductivity. It is hence apparent that the thermal conductivity is never improved by the simple assembly of a wiring pattern of foil, an insulating layer, and a metal substrate.
When the conventional light source apparatus shown in FIG. 24 is intended to emit white light from a combination of the blue LED chip and the fluorescent member, the two regions 33a and 33b should be filled with different sealing resins. The sealing resin 33b contains fluorescent particles dispersed into a resin base for conversion of the blue light into white light. In case that the two sealing resins are identical in the composition, their filling process has to be divided into two steps because one of the sealing resins is mixed with the fluorescent particles. As the two steps are involved, the interface between the two sealing resins 33a and 33b may be developed. More specifically, there are a difference in the thermal expansivity and a difference in the elasticity between the two sealing resins 33a and 33b. Accordingly, when the bonding wire pattern 9 extends across the interface, it may encounter disconnection of its wire.
The present invention has been developed in view of the foregoing drawbacks and its object is to provide a light source apparatus which is improved in the efficiency of light emission to gain the intensity of light output and expand the operational life while is increased in the mechanical strength.